1. Field of the Invention
The present invention relates to an apparatus for reproducing sound, a method for reproducing sound, a monitor with speakers, and a mobile phone with speakers.
2. Background Information
Sound field control technology using a plurality of sound sources has been generally developed in order to improve the acoustic characteristics at some designated positions in space. However, recently, technology has been developing to improve the acoustic characteristics of a specific zone (i.e. where a listener exists). Sound field control technology using a plurality of sound sources can be categorized into two groups in general. One is an active noise control that reduces sound level of a zone by changing the magnitude/phase input to a plurality of sound sources actively. The other is a control that increases the emitted sound power for a specific angle by changing a gap between sources that are arranged in a specific shape [R. C. Jones, “On the theory of the directional patterns of continuous source distributions on a plane surface,” J. Acoust. Soc. Am.16(3), 147-1710 945)] or by changing the time delay and magnitude input between each sound source [R. L. Prichard, “Maximum directivity index of a linear point array,” J. Acoust. Soc. Am. 26, 1034-1039(1954)]. The latter was studied for the active sonar. As representative, Dolph has proposed a mathematical solution having a weight function of a sound source array that generates a sidelobe of constant magnitude so that the emitted acoustic power at a specific directional angle is not affected by the sidelobe [C. L. Dolph, “A current distribution for broadside arrays which optimizes the relationship between beamwidth and sidelobe level,” Proc. IRE 34(6), 335-348 (1946)].
But, it was difficult to apply in the case of an arbitrary source array because it was the mathematical solution for a specific source array. So, the optimization technique for getting maximum emitted sound power to a specific direction in the case of an arbitrary sound source array has been studied by Streit [Roy L. Streit, “Optimization of discrete array of arbitrary geometry,” J. Acost. Soc. Am. 69(1), 199-212 (1981)]. However, this research assumed only an arbitrary sound source array and is not suitable for applying to a common listening space that reflects several acoustic phenomena due to various radiation patterns of sources, reflection/absorption of walls and etc.
The technology controlling sound pressure level in the space where the listener exists has been studied as active noise control [P. Lueg 1936 Process of silencing sound oscillations. U.S. Pat. No. 2,043,416], which is not same with the research optimizing the emission pattern.
The active noise control is a noise reduction method by actively controlling acoustic potential energy or sound power generated by background noise source using second sound sources. It is effective to obtain the silence against the listener or the total space in low frequency range. In this case, the space where the silence is successively gotten by controlling noise is called a quiet zone.
Further, the technology controlling indirect characteristics like directionality by using restrictive assumption that is neglecting the distance to the listener or reflection and the like is known to all by U.S. Pat. No. 5,802,190 (Linear speaker array). The method of reproducing a signal without distortion by using transfer function is also known to all by U.S. Pat. No. 5,910,990 (Apparatus and method for automatic equalization of personal multi-channel audio system).
Consequently, the conventional sound field control methods using a plurality of sound sources are mainly originated by changing the time delay between sound sources and the input magnitude simply, or changing the directionality of emitted sound power by using a restrictive array type of sound source without considering the variable location of the listener or the space where the listener exists. Also, there is the problem that it is not possible to reflect the acoustic characteristics of the listening space due to radiation, reflection, absorption, and so on, because the conventional methods only assume free field condition.